PURPOSE: To treat economically and efficiently salt contg. Na and dust contg. Zn at the same time, by extracting Na from the slag and treating the dust with the resulting NaOH soln. to recover ZnCO3 and Na2CO3.

CONSTITUTION: Slag contg. Na from an iron mill or the like is fed to an extractor 1 to extract Na with water, and after separating the residue, the resulting aqueous soln. is fed to an SiO2 removing vessel 2, where gas contg. CO2 such as exhaust gas from the iron mill is blown into the soln. The soln. is fed to a thickener 3, and precipitated SiO2 is separated and removed to prepare an aqueous Na2CO3 soln. Ca(OH)2 is added to the Na2CO3 soln. in an NaOH producing vessel 4, precipitated CaCO3 is removed with a filter 5, and the resulting NaOH soln. is fed to a Zn dissolving vessel 7. In the vessel 7, dust contg. Zn is added to the NaOH soln., and Fe enriched slurry is separated in a thickener 7, and gas contg. CO2 is blown in a carbonation vessel 8. ZnCO3 obtd. by carbonation is taken out as a slurry in a thickener 9, and an aqueous Na2CO3 soln. is fed to a soda ash recovering apparatus 10.